Unfortunately, sillyd, the frustrating thing is the gasoline definitely will go bad, even if you might be able to snag non-ethanol-containing gasoline (interestingly enough, we can't get gas with ethanol in it in AK), if you let it sit, even in the cool underground, for more than about a month.
I have no qualms in Alaska's climate about letting diesel, aka fuel oil #2, aka heating oil #2, sit for as long as about a year, and propane will last a lifetime. But gasoline...if you absolutely have to store it you can add stabilizer, but that's mighty expensive. The neatest thing to be able to accomplish, if you're trying to stave off the impending apocalypse, would be were you able to foist off - somewhere - the remnant of a 500-gallon tank at the end of each month and then fill it with the fresh stuff. Lather, rinse, repeat. I've absolutely no idea where to suggest you could continuously get rid of a couple of hundred (???) gallons of month-old gas every thirty days, although perhaps you do.
PS: I do store gasoline: I put a couple of hundred gallons in sealed 55-gallon barrels and make use of them over the course of about half a year. The vapors have no way of leaving the fluid and, since there's no ethanol, it doesn't "separate" (and the hygroscopic ethanol can't attract water moisture). But your u/g tank isn't sealed, unfortunately.
PPS: 'Course, if the apocalypse comes and you've those 500 gallons, you'll be good only until you've finished that off...and then the zombies will finish you off anyway, so you're still sunk.
